Best time to buy Manila to Ho Chi Minh or Hanoi return

OK - I picked up a cheapish cent Pacific air ticket a few months ago that will take me to Manila. I want to go to Vietnam on April 29 and return on May 18. Normally I have been able to pick up reasonably priced tickets out of Manila but I've noticed that they are quite a bit pricier than before. I missed out on a 2200 PHP ticket the other day and now the promo fares are 3700 PHP. Should I wait for a better ticket or buy?